article: Animals do many different, amazing things to get through the winter. Some of them _ . They travel to other places where the weather is warmer or where they can find food.
  Many birds migrate in autumn. Because the trip can be dangerous, some travel in large groups. For example, geese  fly in noisy, "V"-shaped groups. Other kinds of birds fly alone.
  Some animals stay active in winter. They must change themselves as the weather changes. Many change their doing or their bodies. For example, snowshoe rabbits grow white fur to help them hide in the snow.
  Food is hard to find in winter. Some animals, like mice, collect lots of food in autumn and store it to eat later. Some animals eat different kinds of food as the seasons change.
  Some animals hibernate for part or all of the winter. This is a special, very deep sleep. The animal's body temperature drops, and its heartbeat and breathing slow down. It uses very little energy. In autumn, these animals get ready for winter by eating much more food than in summer and storing it as body fat. They use this fat to keep them alive while hibernating.
  Water makes good protection for many animals. When the weather gets cold, they move to the bottom of lakes and rivers. There, frogs and many fish hide under rocks or fallen leaves. Cold water holds more oxygen than warm water, and frogs can breathe through their skin.
  Every type of insect has its own life cycle and that is the way it grows and changes. Different insects spend the winter in different forms of their lives. Some insects also spend the winter without moving. Some insects spend the winter as pupae . Other insects die after laying eggs in autumn. The eggs change into new insects in spring and everything begins all over again.
question: Why do many birds fly in large groups?
options: They can easily find food this way. They will use little energy. They can get warm at night. There may be danger during the trip.